1. Field of the Invention
The present invention relates generally to timing systems and more particularly to timing and synchronization systems including an external reference and an internal oscillator.
2. Discussion of the Related Art
A clock is a device for telling time. It consists of three parts: means to set the time; means to advance the time in a controlled fashion; and means to display the time or generate useful outputs, such as time codes, which allow a user to observe or record the time.
Clocks may be mechanical or electronic. The means described here may be implemented in numerous ways; the time may be advanced, for example, using a pendulum, a mechanical escapement mechanism, or a quartz crystal oscillator and electronic frequency divider or counter.
A disciplined clock is a clock in which the means to advance the time, and most commonly also the means to set the time, are provided by one or more inputs for external time reference signals. These inputs might be derived from terrestrial radio time signals, from satellite signals (e.g. GPS or other navigation satellite systems), from a computer network (e.g. Network Time Protocol or NTP), from a locally-available time code (e.g. 1PPS or IRIG-B), or other similar methods, without limitation.
Disciplined clocks commonly contain an oscillator to assist in advancing the time. The more accurate this oscillator (i.e., the closer its rate to the actual rate of time), the more useful it is in maintaining time if the external reference is lost or degraded. An oscillator designed to provide specified accuracy in the event of loss of the external reference time signal is commonly called a holdover oscillator (HO). The HO also provides a backup in the event that the external reference time signal is ‘spoofed’ by a potential adversary.
Lower-quality oscillators may be used, providing the function of a ‘flywheel,’ allowing the clock to interpolate time intervals shorter than that provided by the external reference, and to ride through momentary disruptions. The flywheel oscillator can also provide some holdover capability, though normally with limited performance.
The primary difference between a holdover oscillator and a flywheel oscillator is the time interval over which they are intended to be useful. A flywheel oscillator provides a stable frequency for a short time, perhaps a few seconds or minutes; while a holdover oscillator is intended to provide a usable, stable frequency for periods ranging from hours to days or even months.
A simple quartz crystal oscillator has an accuracy of approximately 10 parts per million (ppm), which means that it might gain or lose 10 microseconds per second, approximately one second per day. This can be adequate for lower-accuracy requirements, for instance a simple wall clock. Technical applications for accurate time often have far more stringent requirements; for these applications, higher-performance holdover oscillators are often required. Examples of higher performance oscillators include temperature-compensated crystal oscillators (TCXO), oven-controlled crystal oscillators (OCXO), and atomic standards (rubidium or cesium oscillators, or hydrogen masers).
Holdover oscillators (and flywheel oscillators) are typically controlled (‘disciplined’) by the external reference, when it is available. This is implemented by means of a tune input to the HO. By adjusting the HO frequency to match the rate of the external reference, the error in holdover can be minimized.
An unintended and undesirable consequence of this is that any errors arising when generating this tuning signal degrade the stability (accuracy) of the holdover oscillator, when the external reference is unusable. The better the potential performance of the HO, the more critical (and costly) minimizing these errors becomes. Furthermore, HO analog-tuning linearity (a factor in accuracy, and more importantly in estimating the HO error) is rarely very good; linearity of +/−10% is typical.
A further, even more serious consequence of the design is that the holdover oscillator becomes a critical failure point of the design. If the HO fails outright, the clock stops. More common is a partial failure, where the HO continues to operate but with errors (time drift) far greater than specified. This might be the consequence, for example, of failure of the oven heater in an OCXO.
Prior art, such as taught by U.S. Pat. No. 8,576,014 to Smiley et al. (hereinafter “Smiley”) discusses selection of different external reference signals based on their performance. Smiley anticipates the availability of a plurality of external references, which may come and go from time to time, and whose accuracy may also vary with time. Smiley teaches a method of selecting between them based on the performance and availability of these references at any point in time.